The present invention is generally related to data handling systems and more particularly to a novel sequential approximation encoding and decoding (SAED) system for transforming analog data into binary signals suitable for storage or transmission and for subsequently reconstructing the original analog data from the binary signals.
Recent progress in data processing technology has created a demand for systems capable of converting high frequency analog data into a digital form more suitable for storage or transmission to a remote receiver utilizing state-of-the-art storage and transmission media. Where commercial TV or radar information is stored using state-of-the-art recording systems, the recorded data must ordinarily be played back using the same equipment used to record it in order that the recorded output have an acceptable degree of time-base stability and tape alignment. However, if the analog information is first converted to digital form prior to being recorded, then buffer devices can be used to electronically correct the time-base instability of the played back signal. Thus, a recording made on one machine can be played back on another machine without unacceptable distortion.
Similarly, in the data transmission arts where large quantities of continuous tone picture imagery or other analog data must be transmitted between remote locations, the continually expanding volume of data to be transmitted has led to further saturation of the already overcrowded electromagnetic spectrum, and now severely taxes systems using present multiplexing techniques.
Most modern analog data transmission systems for transmitting radar video, television signals or scanned photographs are inefficient in the sense that the average source information rate is substantially less than the link channel capacity. The resulting waste can be attributed to at least the following factors:
(1) Where DC response is required, a portion of the transmitter bandwidth must be allocated to insure the effectiveness of the modulation technique utilized.
(2) Where data is obtained from multiple sources, bandwidth in excess of that required to transmit the desired base-band video must be set aside for each source in order to provide adequate guard bands for channel isolation (as in the case of frequency division multiplexing). If time division multiplexing is used, 50% to 80% additional bandwidth must be provided for oversampling to minimize aliasing errors. In addition, presampling filtering must be used which further deducts from the usable video bandwidth.
(3) Where data is obtained from a video source, low communication efficiency stems from the time variant frequency spectral behavior of the source. Since a properly designed communication system must be able to accommodate the highest frequency components anticipated from a given source the system will operate at low efficiency whenever the channel capacity is not completely utilized. A radar base-band video is typical of the time-dependent video signal source.
During the past several years, considerable progress has been made in minimizing the communication inefficiency resulting from these varying signal characteristics through the use of various data compression techniques. Until recently, the most promising compression methods for video bandwidth reduction were the delta modulation (DM) and redundancy reduction (RR) techniques. There have, for example, been more than 100 DM techniques proposed or devised, and several are currently in operational use. For example, see U.S. Pat. Nos. 2,724,740, 2,897,275 and 3,339,142, and the publication by R. M. Wilkinson entitled Delta Modulation for Analog to Digital Conversion of Speech Signals, SRDE Report No. 69022.
In delta modulation, positive or negative binary pulses (marks or spaces) are transmitted at a constant clock rate. The synthesized output wave typically changes one level per clock pulse corresponding to the polarity of the transmitted pulse. The transmitted pulse is positive if the synthesized demodulated output is more negative than the input, and the pulse is negative if the output is more positive than the input. Although delta modulation is simple in terms of circuit complexity, this technique is extremely vulnerable to transmission bit errors and, because it is constrained to change in a fixed step-wise fashion, it typically exhibits poor transient response.
High information delta modulation (HIDM) which is a variation of conventional delta modulation and which has been used for voice transmission can also be used to pulse encode pictorial data. It is able to do this with some degree of efficiency, and requires only two pulses per picture element to provide a subjective quality comparable to fixed bit pulse code modulations. HIDM differs from ordinary delta modulation essentially in the manner of counting amplitude levels. The counting operation in HIDM is in binary steps, and proceeds exponentially for the duration of a sequence of pulses of one polarity. When an overcorrection occurs, pulse polarity reverses and the count direction reverses. When a reversal is required and when the count increment has been large, the sequence does not return to unit count.
In redundancy reduction systems such as disclosed in U.S. Pat. No. 3,383,461, an elaborate process is followed to approximate the input signal waveform by matching polynomials to the input waveform. Redundancy reduction performs extremely well on transient data, but contributes controlled error to the low frequency components, and has not been widely accepted due to data degradation, sensitivity to bit errors and equipment complexity.